fix: only inject portless Node dir into PATH on Windows

[octo-patch]

Fixes #241

Problem

augmentedPath in cli-utils.ts unconditionally prepends path.dirname(process.execPath) to the PATH of every child process spawned by portless. When portless is installed via Homebrew (or any tool that uses its own Node), this places portless's Node binary directory before the user's PATH entries, shadowing the version they've configured through their version manager (nvm/fnm/asdf/mise).

Example from the issue:

$ node --version
v24.13.0           # asdf-managed LTS

$ portless myapp sh -c 'node --version'
v25.9.0            # Homebrew's node, injected by portless

Solution

Restrict the nodeBin injection to Windows only. The comment in the original code already explains it's a Windows-specific need — .cmd wrappers in node_modules/.bin require node to be resolvable via PATH on Windows. On Unix-like systems (macOS/Linux), the user's version manager already places their preferred Node first in PATH, so adding portless's Node directory is both unnecessary and harmful.

Testing

• On macOS/Linux: portless run sh -c 'node --version' now returns the user's version-manager Node, not portless's bundled Node.
• On Windows: behavior is unchanged — .cmd wrappers in node_modules/.bin still find node correctly.
• Existing test suite passes.

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